The use of an electric heating grid to heat automotive window glass is of course known. Such a grid is mounted on, or embedded in, the window glass, and is connected to a control circuit that controls the delivery of electric power to the grid. A typical control circuit comprises an on-off control switch and an associated timer or thermostatic control that is effective to eventually shut off the electric current flow in the grid at some time after the control switch has been turned on. Perhaps the most common application of an electric heating grid to automotive window glass is to the backlight of an automotive vehicle, although it is also known to electrically heat the windshield.
The purpose of heating automotive window glass is of course to defog or defrost the glass so that an occupant of the automotive vehicle can have better visibility through the glass. In the case of an automotive vehicle windshield, it is more typical to have a hot air defroster blow hot air across the inside of the windshield, rather than to employ an electrically heated grid on the windshield. However, each type of defrosting or defogging (i.e., hot air, electric heating) has its own advantages.
Electrical heating will generally result in faster defrosting or defogging, but it requires either an electric heating grid on the windshield or a special windshield containing a special heating element. It also requires that the vehicle's alternator have sufficient capacity to service both the windshield heating load and other electrical loads imposed by the vehicle.
A hot air defroster does not require as large an alternator as is required for an electrically heated windshield, but it does require a longer time to be effective since it derives thermal energy from the engine coolant, meaning that the engine must warm up before the hot air defroster becomes capable of delivering sufficient heat. A hot air defroster may also not be as capable as an electrically heated grid of delivering heat to a particular zone of the glass.
For certain automotive vehicles, a hot air defroster for the windshield may be entirely satisfactory. For other vehicles however the inclusion of electric heating may be desirable, either by itself to the exclusion of a hot air defroster, or else to at times supplement a hot air defroster.
The present invention relates to a new and unique control circuit for an electrically heated windshield which allows the windshield to be electrically heated only under conditions that are truly appropriate for electric heating.
For example, consider the case of a vehicle that is used in the snow. Snow may accumulate in some areas of the windshield which are less efficiently defrosted by the hot air defroster than others. Consequently, those less efficiently defrosted areas may experience a build-up of snow and/or ice that cannot be removed by the action of the hot air defroster alone. Therefore, it becomes desirable to provide those areas with an electric heating grid which is capable of melting snow/ice accumulation, hopefully before such an accumulation ever begins to become significant. However, since such conditions will occur only when the outside temperature is conducive to creating snow and/or ice, it may not be desirable to allow the electric heating to occur when such conditions are not present, and it is toward this objective that one of the several aspects of the present invention is directed.
According to this one aspect of the invention, a temperature sensing switch is associated with the control circuit for the electric heating grid such that the grid is allowed to operate only when the ambient temperature outside the passenger compartment of the vehicle is below a certain threshold. In a preferred embodiment of the invention this temperature sensing switch is disposed on the exterior of the vehicle to sense the outside temperature, and it is connected in a series circuit with an on-off control switch for the electric heating grid. The temperature sensing switch is open above a threshold temperature and closed below the threshold temperature so as to allow the on-off control switch to turn the heating grid on when the outside temperature is below the threshold, and to prevent the on-off control switch from turning the heating grid on when the outside temperature is above the threshold. The temperature sensing switch has a certain hysteresis in its switching characteristic such that it operates from open to closed at a nominal temperature that is below the nominal temperature at which it operates from closed to open. In the disclosed embodiment, the temperature sensing switch operates from open to closed at 40.degree. F. nominal and from closed to open at 50.degree. F. nominal. In this way, the invention allows the electric heating grid to be turned on during what is typically considered winter operation while disallowing the grid to be turned on during what is not considered as winter operation.
Another aspect of the invention relates to the disposition of the electric heating grid on the windshield. Specifically, the grid is disposed to extend from approximately the middle of one side of the windshield down that one side to the bottom of the windshield, and from there across the entire bottom of the windshield. In this way, the grid is for the most part out of the wiping pattern of the windshield wipers where it does not interfere with the view of the vehicle operator; yet it is positioned to be quite effective for melting snow/ice accumulation.
A still further aspect of the invention relates to the specific details of the control circuit. The electric heating grid is connected in series with normally open contacts of a relay whose coil is controlled both by the on-off control switch and by the temperature sensing switch such that the coil is energized to close the normally open relay contacts only when both the on-off control switch and the temperature sensing switch are closed.
The foregoing, as well as additional features, advantages, and benefits of the invention, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a presently preferred embodiment of the invention according to the best mode contemplated at this time for carrying out the invention.